Respiratory syncytial virus (RSV) infects the general population each winter. While RSV infection affects and inconveniences otherwise healthy adults with symptoms of the common cold, it produces especially serious symptoms in infants and young children. Essentially all children are infected with respiratory syncytial virus before the age of 3, and RSV can cause severe bronchiolitis or pneumonia, progressing to morbid disease or death in a significant percentage of children. See K. McIntosh and R. Chanock, Respiratory Syncytial Virus, Chap. 38, in B. Fields et al. (eds.), Virology, Second Edition, Raven Press, Ltd., New York, N.Y. (1990); W. LaVia et al., J. Pediatrics, 121, 503-510 (1992); M. Stretton et al., Pediatr. Pulmonol. 13, 143-150 (1992); and M. Filippell et al., Infection Control 28, 651-671 (1993).
The only drug to be approved by the U.S. Food and Drug Administration (FDA) for treatment of respiratory syncytial virus (RSV) is ribavirin, 1-beta-D-ribofuranosyl-1H-1,2,4-triazole-3-carboxamide. This nucleoside analog was originally selected as a potential antiviral compound effective against RSV by in vitro testing. Such in vitro tests are useful and virtually the only practical method of initially screening and testing potential anti-RSV drugs. A serious drawback of ribavirin is that it must be inhaled as an aerosol which, for infants and young children, means that it must be administered in an enclosed atmosphere. Moreover, ribavirin has toxic side-effects. Thus, there is a need for improved anti-RSV drugs, especially drugs which do not need to be administered by aerosol and preferably can be delivered orally.